A commonly encountered problem in connection with bending sheet material is that the locations of the bends are difficult to control because of bending tolerance variations and the accumulation of tolerance errors. For example, in the formation of the housings for electronic equipment, sheet metal is bent along a first bend line within certain tolerances. The second bend, however, often is positioned based upon the first bend and accordingly the tolerance errors can accumulate. Since there can be three or more bends which are involved to create the chassis or enclosure for the electronic components, the effect of cumulative tolerance errors in bending can be significant.
One approach to this problem has been to try to control the location of bends in sheet material through the use of slitting or grooving. Slits and grooves can be formed in sheet stock very precisely, for example, by the use of computer numerically controlled (CNC) devices which control a slit or groove forming apparatus, such as a laser, water jet, punch press, knife or tool.
Referring to FIG. 1, a sheet of material 21 is shown which has a plurality of slits or grooves 23 aligned in end-to-end, spaced apart relation along a proposed bend line 25. Between pairs of longitudinally adjacent slits or grooves are bending webs or straps 27 which will be plastically deformed upon bending of sheet 21. Webs 27 hold the sheet together as a single member. When grooves that do not penetrate through sheet 21 are employed, the sheet of material is also held together by the web of material behind each groove.
The location of grooves or slits 23 in sheet 21 can be precisely controlled so as to position the grooves or slits on bend line 25 within relatively close tolerances. Accordingly, when sheet 21 is bent after the grooving or slitting process, the bend occurs at a position that is very close to bend line 25. Since slits can be laid out on a flat sheet of material precisely, the cumulative error is much less in such a bending process as compared to one in which bends are formed by a press brake, with each subsequent bend being positioned by reference to the preceding bend.
Nevertheless, even a grooving-based or slitting-based bending of sheet material has its problems. First, the stresses in bending webs or straps 27, as a result of plastic deformation of the webs and slitting at both ends of webs 27, are substantial and concentrated. For grooving, the stresses on the material behind or on the back side of the groove also are substantial and very concentrated. Thus, failures at webs 27 and/or behind grooves 23 can occur. Moreover, the grooves or slits do not necessarily produce bending of webs 27 directly along bend line 25, and the grooving process is slow and inconsistent, particularly when milling or point cutting V-shaped grooves. Grooving, therefore, is not in widespread commercial use.
As can be seen in FIGS. 1A and 1B, if sheet 21 is slit, as is shown at 23a and/or grooved, as shown at 23b, and then bent, bending webs 27a and 27b will experience plastic deformation and residual stress. For slit 23a, of course, material will be completely removed or severed along the length of the slit. For V-shaped groove 23b, there will be a thin web 29 between groove 23b and the convex outside of the bend, but it also will be plastically deformed and highly stressed. The bend for V-shaped grooving will normally be in a direction closing groove 23b so that the side faces come together, as shown in FIG. 1B. Loading of the bent structure of FIGS. 1A and 1B with a vertical force Fv and/or a horizontal force FH will place the bend, with the weakening slits and/or grooves and the plastically deformed straps or webs 27a, 27b, as well as thin web 29, under considerable stress. Failure of the structure will occur at lower force levels than if a non-slitting or non-grooving bending process were used.
Another scheme for sheet slitting to facilitate bending has been employed in the prior art. The slitting technique employed to produce bends, however, was designed primarily to produce visual or decorative effects for a sculptural application. The visual result has been described as “stitching,” and the bends themselves need to be structurally reinforced by beams. This stitched sculpture was exhibited at the New York Museum of Modem Art in 1998 and was designed by two architects, Tehrani and Ponce de Leon. The sculpture is shown described in the publication entitled “Office dA” by Contemporary World Architects, pp. 15, 20-35, 2000. FIGS. 2, 2A and 2B of the present drawing show the stitching technique employed.
As shown in FIG. 2, a plurality of slits 31 are formed in a sheet material 32. Slits 31 are linear and offset laterally of each other along opposite sides of a bend line 33. The slits can be seen to longitudinally overlap so as to define what will become bending straps or “stitches” 34 between the overlapped slit ends. FIGS. 2A and 2B show an enlarged side elevation view of sheet 32, which has been bent along bend line 33 by 90 degrees, and sheet portions 35 and 36 one opposite sides of the bend line are interconnected by the twisted straps or “stitches” 34 which twist or stitch between the 90 degree sheet portions 35,36. The architects of this sculpture recognized that the resulting bend is not structurally strong, and they have incorporated partially hidden beams welded into the sculpture in the inner vertices of each of the stitched bends.
As sheet 32 is bent, straps 34 are plastically twisted over their length with the result that a back side of the strap engages face 38 on the other side of slit 31 at position 37. Such engagement lifts sheet portion 35 up away from face 38 on sheet portion 36, as well as trying to open end 40 of the slit and producing further stress at the slit end. The result of the lifting is a gap, G, over the length of slit 31 between sheet portion 35 and face 38. Twisted straps or stitches 34 force sheet portion 35 off of face 38 and stress both slit ends 40.
Such gaps G are produced at each slit 31 along the length of bend line 33 on alternative sides of the bend line. Thus, at each slit a sheet portion is forced away from contact with a slit-defining face instead of being pulled into contact with the face.
Moreover, and very importantly, the slitting configuration of FIG. 2 stresses each of straps 34 to a very high degree. As the strap length is increased (the length of overlap between the ends of slits 31) to attempt to reduce the stress from twisting along the strap length, the force trying to resiliently pull or clamp a sheet portion against an opposing face reduces. Conversely, as strap length 34 is decreased, twisting forms micro tears in the straps and stress risers and the general condition of the straps is that they are overstressed.
A vertical force (Fv in FIG. 2B) applied to sheet portion 35 will immediately load twisted and stressed strap 34, and because there is a gap G the strap will plastically deform further and can fail or tear before the sheet portion 35 is displaced down to engagement with and support on face 38. A horizontal force FH similarly will tend to crush the longitudinally adjacent strap 34 (and shear strap 34 in FIG. 2B) before gap G is closed and the sheet portion is supported on the opposing slit face.
Another problem inherent in the slitting scheme of FIGS. 2-2B is that the strap width cannot be varied independently of the distance between slits and the strap width cannot be less than the material thickness without stressing the straps to the extreme. When slits 31 are parallel to each other and overlapping, the strap width, by definition, must equal the spacing or jog between slits. This limits the flexibility in designs for structural loading of the straps.
The sheet slitting configuration of FIGS. 2-2B, therefore, can be employed for decorative bends, but it is not well suited for bends which are required to provide significant structural support.
A simple linear perforation technique also was used by the same architects in an installation of bent metal ceiling panels in a pizza restaurant in Boston. Again, the bent sheet components were not designed for, or capable of, bearing significant unsupported loads along the bends.
Slits, grooves, perforations, dimples and score lines have been used in various patented systems as a basis for bending sheet material. U.S. Pat. No. 5,225,799 to West et al., for example, uses a grooving-based technique to fold up a sheet of material to form a microwave wave guide or filter. In U.S. Pat. No. 4,628,161 to St. Louis, score lines and dimples are used to fold metal sheets. In U.S. Pat. No. 6,210,037 to Brandon, slots and perforations are used to bend plastics. The bending of corrugated cardboard using slits or die cuts is shown in U.S. Pat. No. 6,132,349 and PCT Publication WO 97/24221 to Yokoyama, and U.S. Pat. Nos. 3,756,499 to Grebel et al. and 3,258,380 to Fischer, et al. Bending of paperboard sheets also has been facilitated by slitting, as is shown in U.S. Pat. Nos. 5,692,672 to Hunt, 3,963,170 to Wood and 975,121 to Carter.
In most of these prior art bending systems, however, the bend forming technique greatly weakens the resulting structure, or precision bends are not capable of being formed, or bending occurs by crushing the material on one side of the bend. Moreover, when slitting is used in these prior art systems, in addition to structural weakening and the promotion of future points of structural failure, the slitting can make the process of sealing a bent structure expensive and difficult. These prior art methods, therefore, are less suitable for fabricating structures that are capable of containing a fluid or flowable material.
The problems of precision bending and retention of strength are much more substantial when bending metal sheets, and particularly sheets of substantial thickness. In many applications it is highly desirable to be able to bend metal sheets with low force, for example by hand, with only hand tools or with only moderately powered tools. Such bending of thick metal sheets, of course, poses greater problems
In a second aspect of the present invention the ability to overcome prior art deficiencies in slitting-based bending of sheet material is applied to eliminate deficiencies in prior art metal fabrication techniques and the structures resulting therefrom.
A well known prior art technique for producing rigid three dimension structures is the process of cutting and joining together parts from sheet and non-sheet material. Jigging and welding, clamping and adhesive bonding, or machining and using fasteners to join together several discrete parts has previously been extensively used to fabricate rigid three dimensional structures. In the case of welding, for example, a problem arises in the accurate cutting and jigging of the individual pieces; the labor and machinery required to manipulate a large number of parts, as well as the quality control and certification of multiple parts. Additionally, welding has the inherent problem of dimensional shape warping caused by the heat affected zone of the weld.
Traditional welding of metals with significant material thickness is usually achieved by using parts having beveled edges often made by grinding or single point tools, which add significantly to the fabrication time and cost. Moreover, the fatigue failure of heat affected metals is inferior for joints whose load bearing geometries rely entirely on welded, brazed or soldered materials.
With respect to adhesively bonding sheet and non-sheet material along the edges and faces of discrete components, a problem arises from the handling and accurate positioning the several parts and holding or clamping them in place until the bonding method is complete.
Another class of prior art techniques related to the fabrication of three dimensional structures are the Rapid Prototyping methods. These include stereo lithography and a host of other processes in which a design is produced using a CAD system and the data representation of the structure is used to drive equipment in the addition or subtraction of material until the structure is complete. Prior art Rapid Prototyping techniques are usually either additive or subtractive.
The problems associated with subtractive Rapid Prototyping methods are that they are wasteful of materials and time in that a block of material capable of containing the entire part is used and then a relatively expensive high speed machining center is required to accurately mill and cut the part by removal of the unwanted material.
Problems also exist with prior art additive Rapid Prototyping techniques. Specifically, most such techniques are optimized for a very narrow range of materials. Additionally, most require a specialized fabrication device that dispenses material in correspondence with the data representing the part. The additive Rapid Prototyping processes are slow, very limited in the scale of the part envelope and usually do not make use of structurally robust materials.
In a broad aspect of the present invention relating to bending metal sheets, therefore, it is an important object of the present invention to be able to bend sheet material in a very precise manner and yet produce a bend which is capable of supporting substantial loading.
Another object of this aspect of the present invention is to provide a method for precision bending of sheets of material using improved slitting techniques which enhance the precision of the location of the bends and the strength of the resulting structures.
Another object of the present invention is to provide a precision sheet bending process and a sheet of material which has been slit for bending and which can be used to accommodate bending of sheets of various thicknesses and of various types of non-crushable materials.
Another object of the present invention is to provide a method for slitting sheets for subsequent bending that can be accomplished using only hand tools or power tools which facilitate bending but do not control the location of the bend.
Another object of the present invention is to be able to bend sheet material into high strength, three-dimensional structures having precise dimensioned tolerances.
It is another object of the present invention to be able to bend sheet materials into precise three dimensional structures that are easily and inexpensively sealed thus enabling the containment of fluid or flowable materials.
In a broad aspect of the present invention relating to the use of slit-based bending to enhance fabrication and assembly techniques, it is an object of the present invention to provide a new Rapid Prototyping and Advanced Rapid Manufacturing technique that employs a wide range of materials including many that are structurally robust, does not employ specialized equipment other than what would be found in any modern fabrication facility, and can be scaled up or down to the limits of the cutting process used.
It is another object of this aspect of the present invention to provide features within the sheet of material to be bent that assist in the accurate additive alignment of components prior to and after the sheet material is bent.
A further object of the present invention is to provide a fabrication method that serves as a near-net-shape structural scaffold for multiple components arranged in 3D space in the correct relationship to each other as defined by the original CAD design process.
It is a further object of the present invention to provide a method of fabricating welded structures that employs a smaller number of separate parts and whose edges are self jigging along the length of the bends and whose non-bent edges provide features that facilitate jigging and clamping in preparation for welding. In this context it is yet another object of the present invention to provide a superior method of jigging sheet materials for welding that dramatically reduces warping and dimensional inaccuracy caused by the welding process.
Yet another object of the present invention is to provide a novel welded joint that provides substantial load bearing properties that do not rely on the heat affected zone in all degrees of freedom and thereby improve both the loading strength and cyclical, fatigue strength of the resulting three dimensional structure.
Still another object of the present invention is to provide a superior method for:                1) reducing the number of discrete parts required to fabricate a strong, rigid, dimensionally accurate three dimensional structure or assembly, and        2) inherently providing a positioning and clamping method for the various sides of the desired three dimensional structure that can be accomplished through the bent and unbent edges of the present invention resulting in a lower cost, higher yield fabrication method.        
It is a further object of the present invention to provide a method of fabricating a wide variety of fluid containing casting molds for metals, polymers, ceramics and composites in which the mold is formed from a slit, bent, sheet of material which can be either removed after the solidification process or left in place as a structural or surface component of the finished object.
Still another object of the present invention is to provide a sheet bending method that is adaptable for use with existing slitting devices, enables sheet stock to be shipped in a flat or coiled condition and precision bent at a remote location without the use of a press brake, and enhances the assembly or mounting of components within and on the surfaces in the interior of enclosures formed by bending of the sheet stock after component affixation to the sheet stock.
The method for precision bending of sheet material, the fabrication techniques therefore and the structures formed from such precision bending of the present invention have other features and objects of advantage which will become apparent from, or are set forth in more detail in, the accompanying drawing and the following description of the Best Mode of Carrying Out The Invention.